A vehicle engine is traditionally coupled with a transmission for transmitting torque to the wheels via a torque converter. A wet friction interface may be used with the torque converter for establishing torque transfer under a slip condition (i.e., relative rotation of opposing surfaces forming the friction interface). U.S. Pat. No. 6,132,877, issued Oct. 17, 2000 to Winckler et al, assigned to General Motors Corporation, and hereby incorporated by reference in its entirety, describes a friction interface composed of a high density, low porosity, carbon composite material, infiltrated with chemical vapor deposited carbon and, preferably, having a substrate of a satin weave fabric. Another application of a friction interface used in a torque converter is described in U.S. Pat. No.5,195,621, issued Mar. 23, 1993 to Dull et al, assigned to General Motors Corporation, and hereby incorporated by reference in its entirety. The use of friction material having a heat resistant paper support bearing resin-bonded carbon particles is known in the art, as described in U.S. Pat. No. 5,083,650, issued Jan. 28, 1992 to Seiz et al, assigned to Minnesota Mining and Manufacturing Company and to General Motors Corporation, which is hereby incorporated by reference in its entirety.
Additionally, a wet clutch with a friction interface may be used in lieu of a torque converter for vehicle launch with slip, especially on vehicles designed with a wider overall speed ratio, as the ratio-boosting affect of the torque converter may not be necessary in such an application.
In a wet clutch application, at high temperatures or at low relative velocities between the opposing surfaces forming the friction interface (i.e., between a friction member and a reaction plate) there may be an inadequate amount of fluid film separating the opposing surfaces, causing a spike in the coefficient of friction at the friction interface. When this occurs, a portion of the clutch operates in a dry, asperity phase, outside of the desired elasto-hydrodynamic (EHD) phase. Friction modifier additives are typically added to fluid in the transmission to address this problem. The friction modifier additives adhere to the reaction plate, creating a chemical tribological layer with low friction properties on the surface of the reaction plate, and serving to lower the operating coefficient of friction at the friction interface.